Field of the Invention
Mitoxantrone (Mitox), an antitumor 1,4-bis(aminoalkyl-amino)anthracene-9,10-dione, is currently gaining an important place in the clinical management of leukemias and lymphomas as well as in combination therapy of advanced breast and ovarian cancers. Although Mitox is endowed with an improved tolerability profile compared with doxorubicin (DX) and other anthracyclines, this drug is not devoid of significant toxic side effects, especially those associated with myelosuppression and cardiotoxicity. In particular, congestive heart failure is a serious clinical concern in patients previously treated with anthracyclines (for a recent review on the therapeutic and toxicological profile of mitoxantrone see: Faulds, D.; Balfour, J. A.; Chrisp, P.; Langtry, H. D. "Mitoxantrone, a Review of its Pharmacodynamic and Pharmacokinetic Properties, and Therapeutic Potential in the Chemotherapy of Cancer", Drugs 1991, 41, 400-449).
The mechanisms for cellular destruction of Mitox are probably multimodal in their nature: many studies suggest intercalation into DNA as a major cellular event. Nucleic acid compaction and interference with DNA-Topoisomerase II activity, resulting in protein associated-DNA strand breaks have been also proposed as critical events which lead to Mitox induced cell death. Cellular destruction by antitumor anthracene-9,10-diones, including Mitox, has also been attributed to oxidative metabolism which results in the formation of free radicals capable of DNA alkylation and/or DNA scission, yielding non-protein-associated DNA strand breaks. However, it is generally believed that redox-cycling of the quinone moiety is probably more related to the cardiotoxic side effects of Mitox than to the mechanism of its antitumor activity. The cardiotoxicity of Mitox and DX has also been associated with the metal chelating ability of the adjacent hydroxyl and quinone groups. Formation of drug-metal complexes could enhance oxidation-reduction cycling by a metal catalyzed type reaction (Shipp, N. G.; Dorr, R. T.; Alberts, D. S.; Dawson, B. V.; Hendrix, M. "Characterization of experimental mitoxantrone cardiotoxicity and its partial inhibition by ICRF-187 in cultured neonatal rat heart cells", Canner Res. 1993, 53, 550-556).
The significant clinical activity of Mitox makes the development of second generation anthracenedione congeners an attractive area of investigation. To date, much research has been devoted to the exploration of variations in the nature of the side-chains and to the repositioning of the hydroxy substituents and/or the lateral side-chains.
The introduction of heteroatoms in the anthraquinone chromophore is a relatively unexplored approach, but such a change could significantly affect the interaction of the molecules with biological targets. In particular, heterocyclic analogues of anthraquinones a) should basically retain the same spatial and planar characteristics as the parent drugs for host molecular recognition such as DNA intercalation, and b) might introduce additional hydrogen bonding or basic sites, either of which could increase the affinity of the drug for DNA and/or affect the interaction with Topoisomerase II. In addition, the heteroanalogues could be endowed with altered redox properties.
Several aza analogues have been prepared and screened (Krapcho, A. P., "6,9-bis(substituted-amino)benzo[g] isoquinoline-5,10-diones. PCT Intl. Appl. WO 92/15300, Sep. 17, 1992 (herein incorporated by reference); A. P. Krapcho et al.: "6,9-bis[(2-aminoalkyl)aminobenzo[g]isoquinoline-5,10-diones. A novel class of chromophore-modified antitumor anthracene-9,10-diones: synthesis and antitumor evaluation; J. Med. Chem. (1994), in press (herein incorporated by reference).
Among these compounds 6,9-bis[(2-aminoethyl)amino] benzo[g]isoquinoline-5,10 dione dimaleate salt emerged as the most active in antitumor experimental models.
6,9-bis[(2-aminoethyl)amino]benzo[g]isoquinoline-5,10-dione dimaleate salt moreover is devoid of any significant toxic effect on cardiac tissue, after both single and multiple treatment, respectively in the rat and mouse. After single treatment in rat with doses approximately equal to LD.sub.10 and LD.sub.50, the compound 6,9-bis[(2-aminoethyl)amino]benzo[g]isoquinoline-5,10-dione dimaleate salt induced less erythropenia and thrombocytopenia than Mitox. This favorable profile was confirmed in mice after repeated treatments in comparison with Mitoxantrone at equiactive doses on murine leukemia models.
Unfortunately chemical development of the compound faced unexpected problems due to the low level of purity of the compound (purity as low as 96%). Analytical development of the compound moreover revealed the presence of unknown impurities which are formed during the last step of the preparative process and which cannot be removed from the compound with any currently available purification method. Since the unknown impurities account for more than 2%, and one of these unknown impurities alone accounts for 1.3%, the development of the compound is seriously endangered since regulatory authorities require extensive investigations on unknown impurities if these latter are present in such a significant amount.